Anti-skid braking system

ABSTRACT

A vehicle wheel anti-skid apparatus is provided which senses and measures the torque applied to a vehicle wheel, by the road, when the brakes are applied. It uses the wheel torque to power and control the brake thereby limiting the brake friction torque so that it cannot exceed the friction torque capacity of the wheel-road contact, thus maintaining maximum friction coefficients between the wheel and the road and prevent skidding and loss of control of the vehicle.

BACKGROUND OF THE INVENTION

The present invention relates to anti-skid devices and especially to an anti-skid brake for vehicle wheels which utilizes the road-wheel torque to power the wheel brake so that the resisting torque applied by the brake cannot exceed the torque generated by the road on the wheel of the vehicle.

It has been known for some time that, under most road conditions, skidding does not occur suddenly as a result of an instantaneous switch from the coefficient of rolling friction to the coefficient of sliding friction at the wheel-road contact and a simultaneous switch in the brake from a coefficient of sliding friction to a coefficient of static friction. The change from rolling contact to skidding is a gradual, but rapid transition where the coefficient of friction varies with slippage while the brake switches from sliding friction to static friction at the instant the wheel reaches 100% slippage.

Many anti-skid devices sense wheel rotation, vehicle acceleration, or brake pressure to detect a skid condition. Even though some prior art devices measure torque to detect slippage, most of these systems use electronic circuits to pump or pulse the brake pressure causing the tire-road contact to alternate between 100% skidding and free rolling conditions. These prior art systems improve control during braking, but have a number of disadvantages including: (a) not providing optimum stopping conditions, but instead increase stopping distance by operating in the high percentage skid and free roll areas, thus preventing only sustained full skids; (b) their effectiveness is influenced by road conditions since any given wheel rotation rate, vehicle acceleration, or brake pressure could occur at any point in the range of skid; and finally, (c) the systems are complicated by complex electronic-hydraulic circuitry resulting in high manufacturing, installation and maintenance costs in addition to low reliability.

There are also a number of anti-skid devices which sense wheel-road torque and use this to operate servo valves. These similarly have disadvantages such as the brake pressure not being limited at the optimum stopping condition, but merely modified at the 100% skid area, thus permitting possible 100% skids.

The present invention, on the other hand, relates to a combination of devices which are directed toward preventing skidding of a vehicle on any road by adapting the braking force to the existing road wheel friction condition. Thus, the invention will sense the torque applied by the road to the wheel and use this wheel torque to power and control the brake, thus providing a limit on the braking capacity which is dependent on the existing road conditions.

By using the difference in pressure between the pilot brake and the main brake to control and power the brake action, the need for a servo valve is eliminated. The present invention is an improvement over the invention in my prior U.S. Pat. Nos. 3,872,952 and 3,923,345.

SUMMARY OF THE INVENTION

An anti-skid brake mechanism is provided having a fluid drive system for applying fluid pressure to a wheel main brake. A torque measuring pilot brake is provided for actuating the fluid drive system by sensing and measuring a torque created by the road-wheel contact. A converter system is provided for converting the torque measured by the pilot brake into hydraulic pressure. The converter also compares the pilot brake pressure to the main brake pressure and modifies the main brake pressure so that the combined braking effort of the pilot brake and the main brake does not exceed the capacity of the road-wheel interface to maintain maximum friction for existing conditions. The main brake driving system is powered by the hydraulic pressure generated in the converter.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will be apparent from the description and the drawings, in which:

The FIGURE is a schematic sectional view of a preferred embodiment of the present invention having a combination of pilot brake, main brake and converter assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the FIGURE, an anti-skid brake includes a pilot brake assembly A affixed to a specially designed converter assembly B and is hydraulically connected to and operated by the hydraulic pressure generated in a conventional master cylinder 2. A specially designed telescoping compound power converter assembly B consists of two hydraulic cylinders and pistons, one end of which is affixed to the stationary wheel mount 8 and the other end attached to the pilot brake assembly A. The pilot brake assembly is also connected to one of the two cylinders in the converter assembly.

A main wheel brake assembly C is affixed to the stationary wheel mount 8 by means of spring 19 and is hydraulically connected to the other cylinder of the compound power converter assembly B. A rotatable brake disc 18 is rigidly affixed to and rotated with the vehicle wheel in the same manner as conventional disc brakes.

Again referring to the FIGURE, the operator presses on the brake pedal 1 generating a hydraulic pressure P₁ in the master cylinder 2. The pressure P₁ is limited to a predetermined maximum pressure by relief valve 20 and by accumulator 21. The hydraulic pressure P₁ is transmitted through the hydraulic line 3 to the pilot brake cylinder 5 and to the cylinder 9 in the converter assembly B.

The pressure P₁ acting on the area A₁ of the pilot piston 4 causes the pilot brake pad 15 to be pressed into contact with the moving brake disc 18 resulting in a drag or braking force F₁ which is equal to 2P₁ A₁ μ1, where μ₁ is the coefficient of friction between the brake pad 15 and the disc 18. The force F₁ causes the pilot piston assembly A to move in the direction of rotation of the disc 18 thereby compressing spring 10 and energizing seal 12 to prevent passage of fluid from cylinder 7 to cylinder 9, spring 10 and generating a pressure P₂ in the fluid in cylinder 7. The fluid pressure P₂ is communicated through passage 17 to main brake cylinder 13 where it acts against area A₂ of the main brake piston 14, which in turn presses the main brake pad 16 into contact with the brake disc 18 causing a second braking force F₂ which is equal to 2P₂ A₂ μ₁. Note that the relationship between F₁ and the transfer assembly B is such that a second equation for F₁ exists such that:

    F.sub.1 =2P.sub.1 A.sub.1 μ.sub.1 =P.sub.1 A.sub.y +F.sub.s

where F_(s) is the resisting force of spring 10.

Since the equation for decelerating a wheel may be expressed as:

    maR.sub.w =(F.sub.1 +F.sub.2)R.sub.B

Where:

m=mass of load on wheel;

a=the deceleration of the mass;

R_(w) =the radius of the wheel;

R_(B) =the effective radius of the brake assembly; and

F₁ & F₂ are previously described.

It can be easily seen that one equation of the present invention is: E1 ? ##STR1## which shows that the controlled deceleration of a vehicle upon which the brake is installed is directly proportional to the control pressure P₁ as long as a/g is less than μ_(R) where: g is the universal gravitational constant and μ_(R) is the maximum possible coefficient of friction between the wheel and the road for the existing road conditions.

In the case of a hard or panic stop where maximum P₁ is generated, a/g is equal to μ_(R) but skidding is prevented by the fact that overpressuring of P₁ actually causes P₂ to decrease, since P₁ A_(y) opposes P₂ A_(x) so that the total braking effect actually remains at the optimum point, regardless of road conditions. This can easily be seen if Wμ_(R) is substituted for ma in the previous equation which is then rearranged as follows: ##EQU1## where W equals the weight supported by the wheel. It is evident that when a/g is equal to μ_(R), any change in P₁ causes P₂ to change inversely proportional to P₁.

When the brake is applied on any surface, the brake torque T_(B) is proportional only to P₁ since all other factors in the equation are constant. However, the brake torque T_(B) is equal to and limited by the road torque which is equal to μ_(R) WR_(w). As long as μ_(R) does not decrease appreciably, it is not possible for the wheel to skid. If a sudden large decrease in μ_(R) is experienced, such as entering a wet spot or ice with the brakes applied, the brake may momentarily skid. The skid results in a decrease in the road A torque T_(B) and the forces F₁ and F₂. Since both F₁ and F₂ were acting to compress the main spring 19, the main spring 19 is now partially relieved and spring 19 expands, moving the entire main brake assembly C and disc 18 in a direction opposite to the original. Depending on the relationship between areas A_(x) and A_(y), the pressure in the main brake pressure P₂ will be reduced unlocking the entire brake. As soon as the brake is unlocked, the disc 18 and wheel will start to turn in the original direction as a result of the new road force F_(R) which is now made up of μ_(R) where μ_(R) is a lower value than the original. The brake immediately reapplies itself and adjusts to the new road conditions, except now the limiting F_(R) is a much lower value and will prevent skidding on the new, low μ_(R) road surface. The FIGURE shows only one half of the pilot brake and main brake calipers. However, in all equations the caliper halves not illustrated are accounted for by multiplying the areas A₁ and A₂ by two. A compensator 22 is included in the main brake circuit which allows the piston 6 to fully retract in cylinder 7 and uncover seal 11 to recharge the main brake circuit in the event of fluid leakage.

It should be clear that the present invention is not to be construed as limited to the particular forms disclosed herein, since these are to be considered as illustrative rather than restrictive. 

I claim:
 1. An anti-skid braking system comprising in combination:a rotatable wheel; a rotatable disc attached to said rotatable wheel for rotation therewith; a main brake means for resisting the rotation of said rotatable disc and wheel when actuated; master cylinder means for actuating said anti-skid braking system; pilot brake means operatively connected to said master cylinder means for actuation by said master cylinder means to apply frictional braking force against said rotatable disc for braking said wheel, said pilot brake means having at least a portion thereof movable responsive to said rotating disc when said pilot brake is resisting the rotation of said disc; and a power converter means for transferring a braking force from said master cylinder means and from said pilot brake means to said main brake means for applying a frictional braking force to resist said rotatable disc, said power converter means having means for controlling the braking force of said main brake means between the force applied from the master cylinder means and the force applied by the pilot brake means movement responsive to said rotating disc when said pilot brake is resisting the rotation of said disc, said power converter means having a first cylinder connected to said master cylinder means and to said pilot brake means and a second cylinder connected to said first cylinder and to a main brake means cylinder and a sliding piston connected to said pilot brake means for sliding in said power converter means second cylinder to block the connection between said first and second cylinders and to drive fluid in said second cylinder to operate said main brake responsive to said pilot brake moving responsive to said rotating disc, and whereby skidding is prevented in a braking system.
 2. An anti-skid braking system in accordance with claim 1, in which said power converter means includes a pair of pistons, one said piston riding in said first cylinder and having said second cylinder therein and said other piston mounted in said first cylinder and sliding in said second cylinder and said one sliding piston having a set of seals attached thereto, each seal sealing against the flow of fluid in the opposite direction of the other seal and each said seal allowing the flow of fluid thereby in a direction opposite to its sealing direction and one said seal being mounted to said one sliding piston to pass from said second cylinder into said first cylinder when said one sliding piston is fully extended to thereby allow the movement of fluid thereby.
 3. An anti-skid braking system in accordance with claim 2, in which said second cylinder in said one piston has a passageway therefrom coupled to said main brake means cylinder for driving a main brake means piston against said rotating disc.
 4. An anti-skid braking system in accordance with claim 3, in which said power converter means other piston riding in said first cylinder is attached to a stationary wheel mount of a vehicle.
 5. An anti-skid braking system in accordance with claim 4, in which said main brake means is attached to a stationary wheel mount of a vehicle by a spring biased slidable mount.
 6. An anti-skid braking system in accordance with claim 5, in which said power converter means first cylinder has a compression spring therein around said other piston to spring bias said pilot brake means against said one piston.
 7. An anti-skid braking system comprising in combination:brake pedal means for actuation by an operator; master cylinder means operatively connected to said brake pedal means for actuation of a brake system responsive to actuation of said brake pedal means; a pilot brake assembly operatively connected to said master cylinder means for actuation by said master cylinder means to apply frictional braking force to resist a rotating wheel, said pilot brake assembly having at least a portion thereof movable responsive to said rotating wheel when said pilot brake is resisting the rotation of said wheel; a main brake assembly for resisting the rotation of a wheel; and a power converter assembly having a first piston slidable in a first cylinder and second slidable piston slidable in a second cylinder responsive to the movement of at least a portion of said pilot brake assembly to generate in the second cylinder a fluid pressure responsive to the movement of said pilot brake to drive the main brake assembly, said power converter assembly first piston being operatively connected to the second piston for actuation of said main brake assembly by sliding said first and second slidable pistons, whereby the combination of said first and second slidable pistons generate and control the hydraulic pressure in the main brake assembly responsive to force from said rotating wheel when said pilot brake is actuated to resist the rotation of said wheel.
 8. An anti-skid brake system comprising in combination:a fluid actuation means for generating and applying fluid pressure to a brake system; main brake means for braking a rotating brake disc attached to a rotatable wheel; a pilot brake means actuated by and operatively coupled to said fluid actuation means to drive at least one pilot brake pad against a rotating brake disc when actuated by said fluid actuation means and to move pilot brake means with said rotating disc when said pilot brake is actuated thereby; and coupling means operatively attached to said pilot brake means operatively coupling said rotating brake disc to said main brake means when said pilot brake pad is applied against said rotating brake disc for braking said rotating brake disc, said coupling means including a first piston sliding in a first cylinder and a second piston sliding in a second cylinder and said second cylinder being formed in said first piston and said second piston being attached to said pilot brake assembly through said first cylinder, whereby the main brake is actuated and controlled by force from a rotatable wheel. 